buying a used car battery

I need to buy a used car battery, because of cost. ... I know measuring, not voltage, but max amperage a nimh AA can put out is a better indication of health (internal resistance); so, below 1.5 amp max and I can toss it out, as it will refuse to smart charge anyway.

I would expect a lead acid battery (if comparable to nimh) would put out 24 amps on such a test (12 x 2 amps). My meter goes to 10 amps.

How to properly test. (testing voltage before and been burned before).

The method I like to use to test a car battery is first measure static voltage just to see if it isn't already dead. Then measure specific gravity with a hydrometer (assuming the battery isn't sealed) and load test the battery with a hand held battery load tester. If it passes these tests the battery still has some usefull life in it, the question then is how much. The age of the battery could give you some idea, if it is known.

Russel said:

The method I like to use to test a car battery is first measure static voltage just to see if it isn't already dead. Then measure specific gravity with a hydrometer (assuming the battery isn't sealed) and load test the battery with a hand held battery load tester. If it passes these tests the battery still has some usefull life in it, the question then is how much. The age of the battery could give you some idea, if it is known.

Click to expand...

Yes, so you don't get a sulfated battery.

A lot of discarded car batteries are still good because of misdiagnoses.

24 amps would be insufficient.
Depending on the size and type of the motor current while starting can easily reach 300 A for a short duration.

degarb said:

I would expect a lead acid battery (if comparable to nimh) would put out 24 amps on such a test (12 x 2 amps). My meter goes to 10 amps.

Click to expand...

Noooo!!!!!!

Under no circumstances try to do a flash amps test of a car battery. If you are lucky you will only blow the fuse in the meter. If you are unlucky you will explode the meter, cause a fire or burn the house down.

The official way to do a flash amps load test of a car battery is the so called "cranking amps test". This needs a proper battery load tester that a workshop will have. The result for a good battery should be somewhere between 100-200 amps.

However, buying a used battery is a lottery and may be a false economy. Batteries need to be fresh when new, and have a working life of about 5 years. If the battery has been previously discarded, chances are it is no longer working properly and you will have wasted your money. You can buy a crap old battery many times over, or you can spend the same amount on a new battery and look after it. Is cost really that much of a problem?

I went to my local Scrap Metal Dealer to buy some AGM Batteries. I brought along my Voltmeter, but fortunately the manager offered me the use of their Discharge Current tester, so I bought batteries based on good open-circuit voltage and discharge current !

I have a neighbor that did just that 3 times and spent more than a new one, he now buys a new battery at the local xxmart with a 3 yr no questions asked full refund policy and returns them juat before the 3 yrs is up for a new one.

Are you 100% sure it is in fact the battery? Are your battery terminals completely clean? can yo jump start it? what is the running voltage 13.8 or so volts? How much is this used battery? you can get a cheapo at auto zone or sears for like $50-60 may not last too long or be the right size, but will most likely be more reliable then a used battery.

varmint said:

I have a neighbor that did just that 3 times and spent more than a new one, he now buys a new battery at the local xxmart with a 3 yr no questions asked full refund policy and returns them juat before the 3 yrs is up for a new one.

Click to expand...

I know someone who does the exact same thing from that same anonymous department store.

new battery with warranty

Mr Happy said:

This needs a proper battery load tester that a workshop will have.

Click to expand...

Instead of a carbon pile rheostat you can make a 1 kw load resistor from a length of coiled iron coat hanger wire. It has to be immersed in water during the test.

In principle, with a body/skin resistance of 300 to 3000 ohms for most people most of the time, 12vdc should give some people a shock [I>1 mA] but I've never felt a thing. The lowest voltage that anyone ever got electrocuted with [in the US Navy] was 47v.

xul said:

you can make a 1 kw load resistor from a length of coiled iron coat hanger wire

Click to expand...

Only if you know what you are doing. These crazy experiments are all very well, until someone gets hurt.

xul said:

[...] The lowest voltage that anyone ever got electrocuted with [in the US Navy] was 47v.

Click to expand...

I remember something about a Sailor killing himself with a Simpson multmeter powered by a 9V battery.

Resistance is Futile - Darwin Award 1999

I have tested a car battery using a 100 amp load meter and it did not come up bad but when I took it to an auto parts store it was bad and the new battery fixed the issue. I think these big 700+ CCA batteries need about a 300amp load on them to get a decent working check.

Xul you are like some scary crazy MacGyver, LOL.

Russel said:

I remember something about a Sailor killing himself with a Simpson multmeter powered by a 9V battery.

Resistance is Futile - Darwin Award 1999

Click to expand...

In principle 1 joule of energy can be fatal but these defib machines use up to 200 joules.
To get at least 20 mA with 9v you'd need a skin/body of resistance of 9/.02 = 450 ohms. I guess it's possible, but the effect depends on the current path through the body. Through the chest or head is bad.
BTW, the elec. chair comes in at ~2400 VAC/~6A = ~400 ohms.

Regarding
http://en.wikipedia.org/wiki/MacGyver
I have found that my ability to rapidly adapt to a given situation has improved by pretending that the problems of each forum poster on many different forums are actually my problems.

Calculate your risk/benefit ratio, take into account your risk aversion and possible legal liability, don't injure bystanders [because it can turn heros into fools], and go for it!
Sometimes martial arts training helps.

degarb said:

I need to buy a used car battery, because of cost. ... I know measuring, not voltage, but max amperage a nimh AA can put out is a better indication of health (internal resistance); so, below 1.5 amp max and I can toss it out, as it will refuse to smart charge anyway.

I would expect a lead acid battery (if comparable to nimh) would put out 24 amps on such a test (12 x 2 amps). My meter goes to 10 amps.

How to properly test. (testing voltage before and been burned before).

Click to expand...

I would expect up to 1000A depending on the battery. Your multimeter will blow the fuse or it will explode in you hands. If you get a 95Ah AGM battery they all have around 1000A max current.

Never buy a used car battery. It will maybe work a while but you never know what happened to it before.

I can give you a free AGM 95Ah or 62Ah battery but you need to pick it up from Germany. Im a car mechanic so I can get the old ones for free and use them for a few years. Those batteries which can be opened are crap. Most times they are dead when discharged too low. Same for the sealed ones which still have liquid acid inside. AGM is the one you want to get. Those are indestructible! Even when discharged under 1V they are still good after charging.